As well as internal opposition, Venezuela faces mounting international criticism over the election and on Sunday the US said it was considering further sanctions.

As voting got under way on Sunday morning, anti-government protesters took to the streets despite a government ban and there were reports of clashes with police across the country.

At least three people were reported shot dead in the western state of Tachira - two teenagers and a soldier from the national guard.

Ricardo Campos, a youth secretary with the opposition Acción Democrática party, was shot dead during a protest in the north-eastern town of Cumana, prosecutors said.

Image copyrightEPAImage caption
A police officer was wounded in this explosion near the Altamira Square in Caracas

Shortly before voting started, José Felix Pineda, a 39-year-old lawyer standing in the election, was also reportedly shot in his home in Bolivar state.

In the capital Caracas, an explosion near one demonstration injured several police officers and set a number of their motorcycles on fire.

Security forces used armoured vehicles to dispel protesters in the Caracas district of El Paraíso amid the sound of gunfire, local reports said.

Voting was extended by an hour until 19:00 (23:00 GMT), electoral officials said, to allow all votes to be cast.

The opposition urged further protests on Monday.

"We do not recognise this fraudulent process," said opposition leader Henrique Capriles.

Analysis: An uncontested election

Will Grant, BBC News, Caracas

In the capital, voting was peaceful in many neighbourhoods but there were sporadic clashes between protesters and the security forces, especially in traditionally opposition-controlled areas.

With many thoroughfares closed by local citizens erecting barricades of wood, scrap metal and barbed wire, further confrontation seems inevitable.

President Maduro will no doubt claim a resounding victory. But with the opposition boycotting the vote and with the security forces controlling the cities in riot gear it was always going to be an uncontested election.

The challenge will be governing afterwards in this conflicted political environment.

Four months of protests against Mr Maduro and his plans for the assembly have left more than 100 people dead.

The election has been heavily criticised by other Latin American countries as well as by the European Union and the US.

Venezuela has said it will withdraw from the Organisation of American States (OAS) after members including the US, Canada and Mexico said they would not recognise the authority of the assembly.

Venezuela has already been suspended from regional economic bloc Mercosur by fellow-members Brazil, Argentina, Paraguay and Uruguay, following concern over human rights.

On Sunday, the US ambassador to the UN, Nikki Haley, dismissed the vote as a "sham election" and a "step towards dictatorship".

The US, which has already imposed sanctions on 13 members of Mr Maduro's government, signalled on Sunday it was considering further measures, this time targeting the oil industry.

An official quoted by Reuters said the new sanctions were not expected to include a ban on Venezuelan oil shipments to the US but could stop the sale of lighter US crude that Venezuela mixes with its heavy crude and then exports.

Why is the assembly election happening?

Image copyrightAFP

President Maduro wants to form a new constituent assembly that will have the specific purpose of drafting or adopting a new constitution

He said the new assembly would promote "reconciliation and peace", but did not give details of exactly how rewriting the constitution would achieve such broad aims

He also said that the constituent assembly would promote dialogue in the polarised country

The opposition says it is a way for President Maduro to maximise his power and cling on to it for longer, as well as delay future elections

Some loyalists of late President Hugo Chávez are also unhappy, as they treasure the old constitution drafted in 1999